The present invention relates to vibratory material feeders driven in mechanical resonance.
Vibratory feeders exhibit many advantages over other types of material feeding systems such as belt feeders or motor driven auger feeders. These other feeders, while exhibiting excellent long-term control over material feed often have non-uniform material feed over the short term, and are often unacceptable for applications such as ingredient mixing which requires specified material ratios be maintained at all times.
Vibratory feeders are also mechanically simplier than these other types of feeders because they operate without bearings, motor brushes, seals and the like. This results in a higher reliability and lower cost material feeder compared with other types of material feeders, and since vibratory feeders typically have no sparking electrical contacts, they are readily adaptable to highly explosive hazardous environments.
In addition, since the only component of the vibratory material feeder that is in direct contact with the material being fed is a feed tray, or the like, cleaning is greatly simplified. Vibratory feeders also exhibit excellent temperature stability, and are capable of operating with high efficiency, for example, feeding 25 tons of material per hour with only 60 watts electrical input.
However, commercially available vibratory feeders are not without disadvantage. For example, since vibratory feeders depend upon the frequency of mechanical resonance of the feeder, which varies with temperature, feed rates are affected by ambient temperature changes and temperature changes due to warm-up. Feeders driven from the power line exhibit a high degree of sensitivity to line voltage and frequency variation. Also, the material feed rate is often not easily controllable because of the non-linear relationship existing between the actual feed rate and a desired command value, and the fact that the feed rate is not zero based, (i.e., a power greater than zero must be applied to the vibratory feeder before material will begin feeding). This offset changes with type of material and material headload.
Another typical disadvantage of vibratory feeders is that they provide no indication of material feed which makes sensing of clogs and the like very difficult. Also, when materials of different densities are used, or when the feeding tray is changed, tedious mechanical tuning is required in order to reestablish the desired mechanical resonant point corresponding to the drive frequency of the vibratory actuator. Finally, since most feeders are driven with phase control of a voltage derived from a power line, unacceptable electrical interference is generated.
Therefore, while exhibiting many advantages over existing material feeding systems, the vibratory feeder has been limited by the aforementioned disadvantages.
It has been recognized that maximum efficiency occurs when a vibratory feeder is driven at the natural mechanical frequency of the system. However, conventional vibratory feeder systems are typically driven at a frequency different from the natural frequency because large and uncontrollable vibrations may occur unless sufficient damping is provided. For this reason, the conventional system is usually designed so that its natural mechanical frequency is above or below the electrical drive frequency resulting in a corresponding loss of efficiency.
Recently, attempts have been made to intentionally drive vibratory feeders at their natural frequency. However, these systems require electromechanical sensors for detecting the frequency of the mechanical vibration for ultimately controlling the drive frequency, or are driven with non-sinusoidal electrical drive wave forms which may result in undesirable mechanical vibrations at high harmonic frequencies. Thus, these known resonant vibratory material feeders exhibit undesirable mechanical complexity or unpredictable effects of high harmonic vibration, with associated unavoidable material feed rate inaccuracies.